This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Early automatic tachycardia detection systems for automatic implantable cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement) and or the rate of the electrocardiogram to detect hemodynamically compromising ventricular tachycardia or fibrillation.
For example, the 1961 publication by Dr. Fred Zacouto, Paris, France, entitled, "Traitement D'Urgence des Differents Types de Syncopes Cardiaques du Syndrome de Morgangni-Adams-Stokes" (National Library of Medicine, Bethesda, MD) describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram to diagnose and automatically treat brady and tachyarrhythmias. Later detection algorithms proposed by Satinsky, "Heart Monitor Automatically Activates Defibrillator", Medical Tribune, 9, No. 91:3, Nov. 11, 1968, and Shuder et al "Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System", Transactions American Society for Artificial Internal Organs, 16:207, 1970, automatically detected and triggered defibrillation when the amplitude of the R-wave of the electrocardiogram fell below a predetermined threshold over a predetermined period of time.
The initial system proposed by Mirowski et al in U.S. Pat. No. Re 27,757, which relied upon the decrease in the amplitude of a pulsatile right ventricular pressure signal below a threshold over a predetermined period of time, was abandoned by Mirowski et al in favor of the rate and/or probability density function morphology discrimination as described in Mower et al, "Automatic Implantable Cardioverter-Defibrillator Structural Characteristics", PACE, Vol. 7, November-December 1984, Part II, pp. 1331-1334.
Sensing of the mechanical pumping action of the heart in conjunction with loss of ability to pace the heart as a method of fibrillation detection is taught in U.S. Pat. No. 4,144,628, issued to Rizk. Monitoring the intracardiac impedance to detect pumping action in conjunction with ECG analysis is taught as a method of fibrillation detection in U.S. Pat. No. 4,291,699, issued to Geddes et al.
More recently, others have suggested the use of high rate plus acceleration of rate or "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and rate stability (U.S. Pat. No. 4,523,595) to distinguish among tachyarrhythmias. As stated in the article "Automatic Tachycardia Recognition", by R. Arzbaecher et al, PACE, May-June 1984, pp. 541-547, anti-tachycardia pacemakers that were undergoing clinical studies prior to the publication of that article detected tachycardia by sensing a high rate in the chamber to be paced. The specific criteria to be met before attempting tachyarrhythmia termination by pacing involved a comparison of the detected heart rate to a preset threshold, such as 150 beats per minute (400 millisecond cycle length) for a preselected number of beats. As stated above, other researchers had suggested the rate of change of rate or suddenness of onset, rate stability and sustained high rate as additional criteria to distinguish among various types of tachyarrhythmias.
Very generally, the systems that depend upon the aforementioned rate criteria are capable of discriminating tachycardia in greater or lesser degree from normal heart activity but can have difficulty in discriminating ventricular tachycardias from supraventricular tachycardias in some cases or in discriminating stable tachycardias from unstable tachyarrhythmias in which cardiac function is compromised. These difficulties may result in delivery of inappropriate antitachycardia pacing, cardioversion or defibrillation therapies.
Very recently, the concept of employing a physiologic sensor in conjunction with ECG analysis as a method of tachyarrhythmia detection and identification has been revived. Use of pressure sensors is addressed in U.S. Pat. Nos. 4,744,950 and 4,967,749, both issued to Cohen. The use of an oxygen saturation sensor is addressed in U.S. Pat. No. 4,967,748, also issued to Cohen.